Metal stencil printing



, fi 1958 HQNQROSENGREN 3,407,726

METAL STENCIL PRINTING I Original Filed Aug. 15, 1966 V 2 Sheets-Sheet l F g. I '1 v v APPLY ETCH RE8IS- ETCH FOIL l Apg v goggggzP sl lc gf ggffigm TANT LUBRICANT FORCING Lug ac k m SURFACE OF FOIL FILM POSITIVE QE fXE lgfiggfifigfif Fig.3;

I g 1 I Fig 4 INVENTOR Harry N. Rosenglren ATTORNEY *Oct.2 9,1968 H. N. ROSENGRiEN I 3,407,726

METAL STENCIL PRINTING Original Filed-Au 15. 1966 2 Sheets-Sheet 2 INVENTOR- Harry N. Ro sengren J BYH ATTORNEY.

United States Patent 3,407,726 METAL STENCIL PRINTING Harry N. Rosengren, Dallas, Tex., assignor to Electrolytic Machining, Inc., Dallas, Tex., a corporation of Texas Original application Aug. 15, 1966, Ser. No. 572,296. Divided and this application Nov. 3, 1967, Ser. No.

4 Claims. (Cl. 101-4283) ABSTRACT OF THE DISCLOSURE This application constitutes a division of the copending application of Harry N. Rosengren entitled Metal Stencil Printing, filed Aug. 15, 1966, Ser. No. 572,296, and now abandoned.

Screen printing is now generally recognized as being one of the major graphic arts along with letterpress, lithography and gravure, wherein a significant feature of screen process printing is that printing can be made on any material of any size with any color compound of any quantity.

Just a few of the varied items presently being screen printed are closures for liquids, bowling pins, plastic molded signs, promotion folders, banners, glassware, textiles, dresses, book covers, display stands, television and radio panels, toys, wall paper, boxes and cardboard containers, decals, printed circuits and many others.

Screen comprised of silk or metal are most commonly used in screen printing, whereby an emulsion is applied to the screen and removed in a configuration that corresponds to the design to be printed. Many problems are encountered in the use of this type of screen, however. The screen must be cleaned after use to prevent the paint or ink from drying in the openings. The emulsion coating that defines the design is subject to attack and can be dissolved by many cleaning solvents. Pinholes readily develop in the emulsion coating during operation due to wear. Because of the organic nature of the emulsion coating and the silk, the variety of inks and paints that can be used is limited. Moreover, the fabric or screen tends to sag after any substantial use, and registration of screens for multicolor printing is difficult.

The present invention provides a printing stencil comprised of a resilient metal foil thin enough that it is characterized by a high degree of flexibility. A large number of apertures per unit area are formed in the screen conforming to the design or configuration of the pattern to be printed. As a result, a very durable and long lasting screen is provided having the characteristic of a relatively rigid body as compared to a fabric or true screen that sags and stretches, but which is also characterized by a high degree of flexibility for a variety of printing applications and processes. The metal plate itself is inexpensive, the cleaning thereof is simplified and the lifetime of usage greatly exceeds anything heretofore used. Moreover, the metal foil employed lends itself readily to the producing of apertures of uniform size, thereby providing a substantial improvement in the resolution of the silk screen type printing. It will be obvious that such a metal foil stencil is subject to little, if any, wear.

The method of producing the metal foil printing stencil of the invention comprises the steps of applying a photographic resist to one surface of a metal foil, exposing the coated metal surface to light through a film positive which have a fine dot pattern corresponding to the configuration of the pattern to be printed, washing away the unexposed photographic resist to leave remaining on the foil surface an etch resistant lattice work surrounding a myriad of openings exposing the metal foil, and etching the metal foil from one surface to cut apertures through the metal foil.

It is desirable to maintain a high degree of uniformity in the hole sizes so as to increase the resolution of printing of the metal foil stencil. An improved method of the invention comprises the etching of the metal foil from only a single side, lubricating the other side with an etch resistant substance and forcing a flow of air through the aperture from the lubricated side as the apertures are cut through the foil. This causes the lubricant substance to be forced into the hole to stop further etching action and any further enlargement, and to prevent undercutting of the hole. Thus even though some holes are cut through quicker in the etching process than others, the etching action will be stopped upon the hole being etched through, yet will allow the completion of the other apertures.

It has been found that certain foil thicknesses are suitable for the application to screen printing, and also that a range of aperture sizes is necessary. Other important parameters, in addition to those above, will be set forth in the following description.

Many other objects, features and advantages of the invention will become apparent from the following detailed description when taken in conjunction with the appended claims and the attached drawing wherein like reference numerals refer to like parts throughout the several figures, and in which:

FIGURE 1 is a top plan view of a metal foil printing stencil;

FIGURE 2 is a fragmentary enlarged view of one portion of the perforated pattern of the stencil shown in FIGURE 1;

FIGURE 3 is a schematic diagram of the method of the invention;

FIGURE 4 is a fragmentary, side elevational view, in section, of a metal foil stencil during the etching process according to the improved method of the invention;

FIGURE 5 is an enlarged, fragmentary view of the foil and holes during the etching process; and

FIGURE 6 is a side elevational view, in section, of another embodiment for etching a metal .foil according to the method of the invention.

A metal foil printing stencil 20, shown in the top plan view of FIGURE 1, includes a perforated configuration 22 comprised of a myriad of minute apertures formed through the metal foil that conforms to the pattern to be printed. Because of the minuteness of the apertures, the configuration appears as a darkened area conforming to the pattern to be printed, whereas closer scrutiny reveals that the darkened pattern actually is a large number of minute holes 26 formed through the metal foil shown in the enlarged view of FIGURE 2.

The screen is used for printing by mounting the screen to a suitable frame that positions the screen over the work to be printed. Ink or other suitable printing media is then supplied to the top of the screen and the screen is made to contact the work to be printed by any suitable means, such as a squeegee for spreading the ink across the perforated stencil pattern. Excellent separation is achieved between the metal foil stencil and the object being printed, especially because of the resiliency of the foil. Because of the flexibility of the metal foil, however, the screen can be used in a number of applications, including its application to the continuous rotary printing method. In the latter instance, the stencil is mounted on a cylindrical frame and is continuously rotated with the frame onto the work being printed.

' It has been found that a certain range of thickness of the metal foil is necessary for printing purposes. Moreover, certain aperture diameters and mesh range are required. It has been found that steel foil in a thickness range from 0.001 inch-0.0035 inch (0.0254 min-0.0889 mm.) is suitable as the metal foil for the printing stencil. Such foil is readily available from United States Steel, for example. It has been found that foil of thicker dimensions is not suitable as lacking sufiicient flexibility for the screen printing applications, whereas foil of lesser thickness is unsuitable as being too flexible, inadequate rigidity and permanently deformed too easily. Steel foil is preferred over other metals because of its availability and resiliency, whereas it has been found that quite different metal foils, such as pure copper, are unsuitable as being much too malleable and lacking the quality of resiliency that is required.

The diameter range of apertures that have been found to be suitable is from about 0.13 mm. down to about 0.06 mm. for circular holes. This corresponds to a range of 'areas for the individual apertures of from about 0.014 square millimeter down to about 0.003 square millimeter. The mesh range that has been found to be suitable is from about 2,700 holes or apertures per square centimeter to about 6,200 per square centimeter. This gives a range of open area ratios of the foil in the pattern from 0.2 percent for an individual aperture area of 0.003 square millimeter and mesh of 6,200 holes per square centimeter to 0.4 percent for an individual aperture area of 0.014 square millimeter and mesh of 2,600 holes per square centimeter.

In general, better printing resolution is achieved as the hole size is reduced, although as a practical matter the hole size can be reduced to no more than a minimum and still cut the holes precisely and force printing ink through the holes. Use of hole sizes greater than the above stated range causes a loss of resolution in printing and nonuniformity of the ink deposited on the object to be printed. Loss of resolution in the use of too large holes becomes more apparent as the edges of the patterns are scrutinized, wherein the printed pattern edges begin to take on the shape of the hole rather than a shar line boundary.

The method of producing the metal foil printing stencil is shown schematically in the block diagram of FIG- URE 3. The metal foil is initially cleaned of all traces of dirt and foreign matter by any suitable method. Thereafter, the foil is coated, as in step 30, on one surface with a photographic resist, for example, Kodak Thin Film Resist (KTFR). This resist is thinned by fifty-percent with Kodak Metal Etch Thinner prior to application. One method of coating is to pour the resist onto the metal foil, move the metal foil about and then allow the resist to drain from one corner. The foil can also be Whirler coated with the resist, as is well known. This will give a substantially even and smooth coat of the resist. Many other photographic resists are available and suitable for this purpose. After the foil has dried, the coated surface of the foil is exposed to a film positive that has a fine opaque dot pattern corresponding to the configuration of the pattern to be printed. Any suitable method can be used to produce the positive, such as, for example, to overlay a film negative of the pattern to be printed with a screen pattern and take a contact film print to produce the positive. The result is a clear film except where the dots appear. The coated metal foil surface is then exposed, as in step 32, by any suitable means, such as by shining a light through the film positive to expose the coated surface of the foil except for the opaque dots forming the pattern.

In one instance, a 133 line 20% screen pattern was used, which is a standard expression that indicates that there are 133 dots to a linear inch with the dots occupying 20% of the total area. A screen of this type has 17,689 dots per square inch. This yields individual dot diameters of 0.09 millimeter, or area of 0.007 square millimeter. Because of the minuteness of the dot size, and the relative thickness of the photographic resist applied to the foil surface, exposure can be critical and should'be done with care. Proper exposing is accomplished by under-exposing the dot pattern with the positive and reducing the developing time. Thereafter, the film is re-exposed to light and the foil then heated at about 275 F. for about 10 min? utes to set the resist.

For example, a normal exposure would require about five minutes, with a subsequent immersion in Kodak Metal Etch Developer for two to three minutes. The foil is then rinsed in water to remove the unexposed resist dot pattern. Preferably, however, the resist is exposed for not more than one minute, immersed in a developer for about fifteen seconds, rinsed in water and then re-exposedfor a full five minutes.

The next step 34 of the process is to coat the back side of the metal foil with a lubricating substance that is etch resistant. Vaseline petroleum jelly has been found tobe a suitable lubricant for this purpose, wherein a fairly heavy coat of Vaseline is applied by any suitable means, such as the finger.

The metal foil is then secured at the lubricated back surface to a metal plate which has an opening in the center. The metal backing plate is connected to one terminal of a D.C. voltage supply. The foil and metal backing plate are then immersed in an electrolyte just above another electrode plate to which the other terminal of the D.C. power supply is connected, so that the metal foil faces this electrode plate in the etch solution. The-metal foil is then etched, as denoted in step 36, for a suitable length of time to etch holes therein which correspond to the areas where the photographic resist was washed or rinsed away. It will be recalled that these holes correspond to the opaque hole or dot pattern of the photographic positive. As a hole is etched through the metal foil, the Vaseline or lubricant is forced by air or fluid flow into the hole to coat the sides thereof and to prevent further etching of this particular hole. Each area which is subject to the etch continues to be etched until the hole breaks through the metal foil, at which time the etch is stopped with the Vaseline is forced into the hole.

Any aperture configuration can be used, such as a square, rough, oval, rectangular, etc. For most printing applications, circular apertures, or holes, are quite suitable.

A suitable etchant for steel foil is a saturated solution of three parts sodium chloride and one part ammonium chloride. The metal foil to be etched is faced downward just above the negative electrode plate, say about 0.5 inch, with etching allowed to proceed at about four volts difference between the two electrodes. Complete etching of all of the holes usually requires about five minutes.

Referring to the fragmentary, side elevational view, in section, of FIGURE 4, the metal foil 20 with a lubricant or Vaseline coating 40 applied to one side thereof is secured to a metal backing plate 42 having a large opening 44 therethrough. A tubing 46 is inserted in the hole 44 through which air is forced. The metal foil and backing plate is immersed in an electrolytic etch solution 48 just above a negative metal electrode 50 to which is connected to the negative terminal of a DC power supply through connection 54, with the backing plate 42 being connected to the positive terminal of the DC. power supply through connection 52. The exposed and hardened photographic resist coating 55 is previously formed on the bottom side of the metal foil as described above, and defines a pattern of openings 56 (resist that was not exposed and removed) that corresponds to the holes to be etched and the pattern to be printed. 0

Because of the slight variances of the electrical resist-. ance and other physical properties of the metal foil, etching does not necessarily proceed uniformly at each exposed area. Thus some holes 26 will be cut through the metal foil before other holes 57 can be completely cut through, as is seen more clearly in the enlarged, fragmentary view of FIGURE 5. It is for this reason that air and Vaseline are used. As a hole is formed, air pressure immdeiately pushes Vaseline into hole 26 to form a film 58 on the inside Walls thereof, and etching stops Within this hole. This prevents enlargement and undercutting of the hole as other holes are etched. As each hole is broken through the foil, this action takes place, with the result being uniform holes throughout the stencil configuration. This not only insures uniform hole size for such minute etching operations but also prevents destruction of the latticework between the holes which would ordinarily result with uncontrolled etching.

After etching has been completed, the foil is removed, and the exposed and hardened photographic resistis removed with a suitable solvent. The stencil is then ready for printing.

Another arrangement for etching the metal foil according to the method of the invention is shown in the side elevation view in section, of FIGURE 6. A film 4ft of etch resistant lubricant or fluid is again applied to one side of the metal foil 20, andthe foil is placed down on an electrode plate 60 with the side having the lubricant abutting the electrode. A relatively large aperture or hole 62 is provided in this electrode over which a part of the foil spares. A metal pipe or tubing 46 is disposed above the foil that spans aperture 62 in relatively close proximity thereto, with tubing 46 being connected by any suitable means (shown schematically) to a pump 64. The liquid etching electrolyte within which the tubing, foil and electrode 60 are immersed is drawn upward through tubing 46 by the pump and returned to the etching container (also shown schematically). Tubing 46 acts as the other electrode and is connected to the negative terminal of the volt-age supply (not shown), whereas electrode 60 is connected to the positive terminal of the supply.

As a hole apertured is etched completely through the metal foil from the top side, the Vaseline or other etch resistant lubricant is forced up into the aperture against the walls thereof by the flow of liquid electrolyte through the aperture caused by the pump. This stops the etching in this particular hole but allows the continuing etching of the holes that have yet to break through the foil. In addition, the flow of electrolyte through these holes that have broken through removes hydrogen bubbles that form on the negative electrode that could ordinarily impede the etch process, and also removes metal particles from the holes, thus further facilitating the process.

It will be understood that this arrangement can be inverted so that gravity aids in facilitating the removal from the foil of particulate matter, or the arrangement may be disposed in any position in between as desired.

The etch process has been described as being electrolytic. However, electroless acid etching can also be used.

Certain modifications and substitutions will undoubtedly occur to those skilled in the art, although it is not intended that the invention necessarily be limited to the specific embodiments described. Rather, it is intended that the invention be limited only as defined in the appended claims.

What is claimed is:

1. A method of making a printing stencil from a metal foil, comprising the steps of:

(a) applying a photographic resist to a surface of said foil,

(b) masking a multiplicity of individual areas of said resist while exposing the resist surrounding said multiplicity of individual areas,

(c) developing the exposed resist and removing the unexposed resist to expose said foil at said multiplicity of individual areas,

(d) lubricating the opposite surface of said foil with an etch resistant fluid, and

(e) etching said foil at said individual areas to make perforations therethrough While forcing said etch resistant fluid under pressure into said perforations as they are formed.

2. A method as set forth in claim 1 wherein said etch resistant fluid is forced into said perforations by forcing a flow of air against said opposite surface.

3. A method as set forth in claim 1 wherein said etch resist-ant fluid is forced into said perforations .by forcing a liquid through said perforations.

4. A method of etching substantially uniform sized perforations in a metal body, comprising the steps of:

(a) applying an etch-resistant lattice work to a first surface of said body,

(b) applying an etch resistant fluid to the opposite surface of said body through which said perforations are to be formed, and

(c) etching said body from said first opposite surface to form said perforations while forcing said etch resistant fluid under pressure into said perforations as they are for-med.

References Cited UNITED STATES PATENTS 2,569,752 10/1951 Fowler 101-l28.3 3,202,094 8/1965 Smallrn'an 101-1282 DAVID KLEIN, Primary Examiner. 

